Current and projected nest site availability for cavity-nesting waterfowl in the Upper Mississippi River and Great Lakes Region.

摘要

Clearing of old growth forests resulted in a substantial loss of nesting habitat for cavity-nesting waterfowl during the late 19 th and early 20th centuries. Since then, many hardwood forests have matured into size classes capable of producing cavities suitable for nesting ducks. To quantify changes in cavity availability in U. S. Fish and Wildlife Service Region 3, which contains most of the midwestern U.S., I examined current cavity availability at 4 sites where cavity availability had been estimated in the past; Mingo National Wildlife Refuge (NWR) in Missouri, Shiawassee NWR in Michigan, Mead Wildlife Area (WA) in Wisconsin, and Muscatatuck NWR in Indiana. I found densities of 1.8 +/- 0.4, 1.4 +/- 0.3, 0.9 +/- 0.4, and 1.8 +/- 0.4 suitable cavities per hectare at each of these sites, respectively. Suitable cavities per hectare increased at Mingo NWR (433%) since 1966 and Shiawassee NWR (1400%) since 1974, but remained similar at Mead WA and Muscatatuck NWR since the mid-1980's, after accounting for differences in past, study-specific criteria for cavity suitability. Differences among sites were likely due to variation in species composition, stage of forest maturation, timber management, and time elapsed since the previous studies. Comparison of size-class distributions for all trees and for trees with cavities indicated that cavities occur in the largest trees and that forests have yet to mature into the most prolific cavity-producing size classes. This conclusion was corroborated by forest growth modeling results from Forest Vegetation Simulator, a forest growth-modeling program from the USDA Forest Service. I used Forest Inventory and Analysis data to model growth from 2008 to 2058 at 10-year intervals for Region 3. Cavity per tree estimates from the 4 study sites were applied to modeling outputs, and cavity availability was projected to almost double over the entire region by 2058. Thus, the observed and further expected increases in cavity density in the region justify re-examination of nest box programs and possibly a reduction of artificial nest boxes in some areas of the region where sufficient natural cavities exist. Because current and future estimates indicate sufficient nest sites for cavity-nesting waterfowl, efforts should be spent protecting and restoring brood-rearing wetlands, which are known to be declining in many areas of Region 3.